<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        li {
            list-style: none;
            float: left;
            width: 30px;
            height: 30px;
            background-color: pink;
            border: 1px solid #ccc;
        }
    </style>
</head>

<body>
    <ul>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
        <li></li>
    </ul>
    <script>
        var lis = document.querySelectorAll("li");
        for (var i = 0; i < lis.length; i++) {
            lis[i].id = i;
            lis[i].onclick = function() {
                console.log(this.id);
            }
        }
    </script>
</body>

</html>